The housing of a refrigerator recently used is one formed as a thermal insulation housing manufactured by a method wherein a space between an internal box and an external box which has been constructed by combining the two boxes is filled with a foam produced by injecting, foaming and solidifying a rigid polyurethane foam stock within the space (referred to hereinafter as in-situ foaming method). Also in the door of the refrigerator, a rigid polyurethane foam is charged as a thermal insulator by the in-situ method between the space which has been constructed by combining the internal and external boxes.
Hitherto, an ABS resin widely used for various purposes has been primarily used as the internal box of a refrigerator. The phraseology "ABS resin widely used for various purposes" means a resin composition which is obtained by polymerizing a monomer mixture of 10-40% by weight of a vinyl cyanide compound and 60-90% by weight of an aromatic vinyl compound in the presence of a poly(conjugated diene) synthetic rubber or, in other words, by graft copolymerizing the monomer mixture; or a resin composition in which a copolymer of a vinyl cyanide compound/aromatic vinyl compound is blended with the graft copolymer so that the content of the vinyl cyanide compound is in the range of 10-40% by weight.
As the reason why an ABS resin has been used as a material of an internal box of a refrigerator, there are mentioned a high balance of physical properties such as rigidity and impact property, easy formability, excellent glossy appearance and stress cracking resistance against a flon such as CFC-11 (trichloromonofluoromethane) which is a foaming agent for a rigid polyurethane foam. The internal box of a refrigerator is produced by a heat forming method of a thermoplastic resin sheet, for example by a vacuum forming technique, so that the sheet should have easy formability, particularly vacuum formability. The internal box obtained by the vacuum forming has an average thickness of less than 1 mm, and thus it should have high modulus in order to avoid deformation. If a rigid polyurethane foam is charged into a space which is formed by combining an internal box made of the ABS resin with an external box made of a steel sheet by the in-situ method, the rigid polyurethane foam adheres to the ABS resin and the steel sheets thus causing stress attributed to the difference in the linear expansion coefficients of the steel sheet/rigid polyurethane foam/ABS resin during the operation of the refrigerator. Therefore, the internal box should have stress cracking resistance against CFC-11 as a foaming agent for the rigid polyurethane foam. As notch effect is produced by the adhesion of the rigid polyurethane foam, the internal box is required to have a high notched Izod impact value. Furthermore, excellent gloss is required in order to improve the appearance of the refrigerator.
CFC-11 as a foaming agent for a rigid polyurethane foam contains chlorine and is non-decomposable. Thus, it is suspected that it can destroy the ozone stratum in the stratosphere and there is thus a worldwide trend to the restriction of its use. As an alternative to CFC-11 for use as a foaming agent for a rigid polyurethane foam to, another fluorinated/chlorinated hydrocarbon such as HCFC-123 (1-hydro-1,1-dichloro-2,2,2-trifluoroethane) is now to be used. However, HCFC-123 has by far a higher solubility of an ABS resin therein than CFC-11, and the internal box of a refrigerator made of the ABS resin in which a rigid polyurethane foam has been charged by the in-situ method will easily produce craze or crack under stress to lose the article value of the refrigerator. Therefore, under such a trend, it is necessary to prepare an internal box of a refrigerator and the interior of a refrigerator door with a material which has a stress cracking resistance against HCFC-123, and in the same time as usual has a high balance of physical properties such as rigidity and impact property, easy formability and excellent glossy appearance.